Direct-Current Coupling System and Charging Control Method Therefor

The present application claims priority to Chinese Patent Application No. 202410772072.1, titled “DIRECT-CURRENT COUPLING SYSTEM AND CHARGING CONTROL METHOD THEREFOR”, filed on Jun. 14, 2024 with the China National Intellectual Property Administration, which is incorporated herein by reference in its entirety.

The present disclosure relates to the technical field of photovoltaic power generation, and in particular to a direct-current coupling system and a charging control method for the direct-current coupling system.

Generally, a direct-current side of a photovoltaic inverter is only connected to a photovoltaic panel, and thus there is only a discharging scenario where the direct-current side of the photovoltaic inverter transmits power to the power grid without considering a charging scenario where the power grid transmits power to the direct-current side of the photovoltaic inverter. In a direct-current coupling system, that is, a direct-current side of an inverter is connected to the photovoltaic panel and an energy storage system, the power grid may transmit power to the direct-current side of the inverter. In such case, both the photovoltaic panel and the power grid may charge the energy storage system.

However, a charging strategy for the direct-current coupling system is currently only applicable to a scenario of a lower requirement on a charging power and a response speed at an alternating-current side of the power grid, thus limiting the application scenario of the direct-current coupling system.

In view of this, a direct-current coupling system and a charging control method for the direct-current coupling system are provided in the present disclosure, to improve the charging power and the response speed, to maximize the power response capability at the alternating-current side of the inverter, and to expand the application scenario.

In order to achieve the above objectives, the following technical solutions are provided according to the present disclosure.

In a first aspect, a direct-current coupling system is provided in the present disclosure. The direct-current coupling system includes at least one inverter, at least one photovoltaic array, and at least one energy storage system, where an alternating-current side of the at least one inverter is configured to connect a power grid; and a direct-current side of the at least one inverter is connected to the at least one photovoltaic array and the at least one energy storage system through a corresponding direct-current bus; the at least one energy storage system includes a battery system and a direct-current/direct-current (DC/DC) converter, and the battery system is connected to a direct-current bus through the DC/DC converter; and the DC/DC converter is configured to acquire, in response to a power regulation demand at the alternating-current side, energy from the power grid through the inverter connected to the DC/DC converter during charging the battery system.

In a second aspect, a charging control method for a direct-current coupling system is provided in the present disclosure, applied to any direct-current coupling system according to the first aspect, where the at least one photovoltaic array in the direct-current coupling system is connected to the at least one inverter during charging the battery system, and during charging the battery system, the charging control method includes: determining, by the at least one inverter, a second voltage for a charging balance of the battery system and a first voltage in a real time manner, and sending, by the at least one inverter, the second voltage to respective DC/DC converter connected to the at least one inverter, where the first voltage is greater than the second voltage; controlling the at least one inverter to operate on condition that the first voltage serves as a predetermined bus voltage value of the inverter; and controlling the DC/DC converter to operate on condition that the second voltage serves as a predetermined bus voltage value of the DC/DC converter.

In a third aspect, a charging control method for a direct-current coupling system is provided in the present disclosure, applied to any direct-current coupling system according to the first aspect, where the at least one photovoltaic array in the direct-current coupling system is connected to the direct-current bus through a direct-current switch, and during charging the battery system, the charging control method includes turning off the direct-current switch by the at least one inverter in the direct-current coupling system; and controlling by the at least one inverter, a bus voltage of the inverter in a constant voltage technology tracking mode.

The technical solutions according to the embodiments of the present disclosure are described clearly and completely hereinafter in conjunction with the drawings in the embodiments of the present disclosure. It is apparent that the described embodiments are only some of the embodiments according to the present disclosure, rather than all the embodiments. Any other embodiments obtained by those skilled in the art based on the embodiments of the present disclosure without any creative work fall within the protection scope of the present disclosure.

The terms “include”, “comprise” or any other variants thereof are intended to be non-exclusive. Therefore, a process, method, article or device including a series of elements include not only these elements but also other elements that are not clearly enumerated, or further include elements inherent in the process, method, article or device. Unless expressively limited, the statement “including a . . . ” does not exclude the case that other similar elements may exist in the process, method, article or device including the series of elements.

As the large energy base becomes a major support of renewable energy development, an application of a direct-current coupling photovoltaic storage joint project with flexible expansion is continuously increased.illustrates a block diagram of a direct-current coupling system. A battery system (represented by Battery shown in) is connected to a direct-current side of an inverter (represented by PV Inverter shown in) through a DC/DC converter (represented by DC/DC shown in). A photovoltaic array is directly connected to the direct-current side of the inverter. An alternating-current side of the inverter is configured to connect the power grid. The inverter is communicatively connected to the DC/DC converter through CAN communication. The inverter and the DC/DC converter are communicatively connected to a local controller (represented by LC shown in). Where Vrepresents a voltage of the direct-current side of the inverter, Prepresents power transmitted between the DC/DC converter and the direct-current side of the inverter, Prepresents power transmitted from the photovoltaic array to the direct-current side of the inverter, and Prepresents power transmitted between the alternating-current side of the inverter and the power grid.

In the structure shown in, during charging the battery system, the DC/DC converter may acquire energy from at least one of the photovoltaic array and the inverter. In a case that the energy is preferentially acquired from the photovoltaic array, if the battery system fails to accommodate grid power provided by the inverter when frequency regulation occurs, the frequency regulation may fail. In a case that the energy is proportionally acquired from the photovoltaic array and the inverter, the inverter may not transmit charging power in response to the frequency regulation in a real time manner. Therefore, the above solution is not applicable to a scenario where the charging power and the charging speed at the alternating-current side of the inverter are highly required, for example, Australian Frequency Control Ancillary Services (FCAS) markets.

In addition, the above solution fails to timely regulate the energy acquired from the power grid based on a change in an electricity price of the power grid, resulting in high charging cost.

The control of the charging power and the charging speed of the alternating-current side of the inverter directly affect the frequency regulation capability and the charging cost of the system. Therefore, in order to improve the charging power and the response speed at the alternating-current side of the inverter, to maximize the power response capability of the alternating-current side of the inverter, and to expand the application scenario, a direct-current coupling system is provided according to the present disclosure. As shown in, the direct-current coupling system includes at least one inverter, at least one photovoltaic array, and at least one energy storage system.

An alternating-current side of the inverteris configured to connect a power grid. In practice, the alternating-current side of the invertermay be connected to the power grid through a step-up transformer.

A direct-current side of the inverteris connected to the at least one photovoltaic arrayand the at least one energy storage systemthrough a direct-current bus.illustrates an example in which the direct-current side of each inverteris connected to two photovoltaic arraysand one energy storage system. In practice, the number of the photovoltaic arrayconnected to each inverterand the number of the energy storage systemconnected to each invertereach are not limited, depending on the application environment.

It should be noted that, in practice, the number of the invertermay be one or more. As shown in, the number of the inverteris more than one, each invertermay independently operate. Alternatively, as shown in, two invertersmay be integrated into one device, and each of the invertersserves as an inverter unit in the device, and direct-current sides of the invertersare connected in parallel in the device, that is, the invertersshare a same direct-current bus.illustrates an example in which two invertersare integrated. In practice, more invertersmay be integrated, which is not limited here, depending on the application environment.

In addition, the energy storage systemincludes a battery systemand a DC/DC converter, and the battery systemis connected to a direct-current bus through the DC/DC converter. Generally, the DC/DC converterreceives a power direction and a voltage sent by the corresponding inverter, and operates based on the power direction and the voltage, achieving charge and discharge of the battery system. The DC/DC converteris communicatively connected to the corresponding inverter. In practice, the DC/DC converterand the invertermay be communicatively connected to the local controller LC.

A frequency regulation process of the power grid or a process of timely regulation of the energy acquired from the power grid based on the electricity price of the power grid is equivalent to a power response process at the alternating-current side of the inverter. Therefore, in this embodiment, the DC/DC converteracquires energy from the power grid (simply referred to as power grid energy) through the inverter connected to the DC/DC converter to charge the battery systemon condition that the DC/DC converterand the inverterrespond to a power regulation demand at the alternating-current side of the inverter. The power regulation demand at the alternating-current side refers to a frequency regulation demand or a regulation demand of the energy acquired from the power grid based on the electricity price of the power grid.

In practice, in a case that the power at the alternating-current side of the inverterresponding to the power regulation demand at the alternating-current side is less than or equal to a charging power of the battery system, the power regulation demand at the alternating-current side is met during charging the battery system. In a case that the power of the alternating-current side of the inverterresponding to the power regulation demand at the alternating-current side is greater than the charging power of the battery system, the power grid energy is used for responding to the power regulation demand at the alternating-current side, which can greatly meet the power regulation demand at the alternating-current side. That is, compared with preferentially using the energy outputted by the photovoltaic array, in this embodiment, the DC/DC converteris configured to preferentially acquire the power grid energy, maximizing the power grid energy accommodated by the battery system, thereby greatly responding to the power regulation demand at the alternating-current side of the corresponding inverter. For each energy storage systemshown inand, the DC/DC convertermay preferentially acquire the power grid energy outputted by the inverterconnected to the DC/DC converter to charge the battery systemof the energy storage system.

In the direct-current coupling system according to the embodiment, the battery systemis charged preferentially by using the power grid energy outputted by the inverterin a case that the DC/DC converterreceives energy from at least one of the inverterand the photovoltaic array, maximizing a power regulation margin at the alternating-current side of the inverter, that is, maximizing the power response capability at the alternating-current side of the inverter, meeting a higher requirement on the charging power and the response speed, for example, responding to the frequency regulation demand in a real time manner, or achieving off-peak charging, optimizing the charging time of the battery system, and reducing the charging cost of the system, thereby expanding the application scenario.

Based on the above embodiment, in this embodiment, an example that the DC/DC converterin the direct-current coupling system charges the battery systempreferentially by using the power grid energy is described in detail. For example, during charging the battery system, if the energy acquired from the power grid is not sufficient for the energy required by the battery system, the DC/DC converteracquires remaining energy required by the battery systemfrom the photovoltaic array. The energy acquired from the power grid refers to energy acquired from the power grid by the inverterconnected to the DC/DC converterin response to the power regulation demand at the alternating-current side.

That is, the DC/DC converterpreferentially acquires energy from the inverterin response to the power regulation demand at the alternating-current side, and the remaining energy required by the battery systemis provided by the photovoltaic array. For example, if the energy from the power grid by the inverterin response to the power regulation demand at the alternating-current side is sufficient for the energy required by the battery system, no power is outputted from the photovoltaic array. Only if the power grid energy provided by the inverterin response to the power regulation demand at the alternating-current side is not sufficient for the energy required by the battery system, the photovoltaic arrayoutputs power to provide the remaining energy. The inverterprovides power to charge the battery systemin response to the power regulation demand at the alternating-current side such as the frequency regulation demand in a real time manner. In this case, if the battery systemfurther needs large power, the photovoltaic arrayoutputs relatively large power, to supplement the remaining power. If the battery systemfurther needs small power, the output power of the photovoltaic arrayonly needs to meet a small power demand. If the power provided by the inverterjust meets the charging demand of the battery system, that is, the battery systemno longer needs a power supplement, no power is outputted from the photovoltaic array.

In practice, both the inverterand the DC/DC converterhave respective voltage loops, and a direct-current bus voltage (simply referred to as a bus voltage) may be regulated by the two voltage loops, that is, both references of the two voltage loops are respective predetermined bus voltage values. Therefore, in order to achieve the above functions, the two predetermined bus voltage values compete for the control of the bus voltage. In an embodiment, the predetermined bus voltage value of the inverteris set to a value X, and the predetermined bus voltage value of the DC/DC converteris set to a value Y, where the value X is greater than the value Y.

In a case that the battery systemcompletely absorb the energy of the inverter, and maximum power currently outputted by the inverterfails to meet the demand of the battery system, the inverteris in a constant power saturation state due to X>Y, and fails to regulate the bus voltage to the predetermined bus voltage value X of the inverter, instead, the bus voltage is controlled by the DC/DC converter. That is, the direct-current bus voltage is controlled by the predetermined bus voltage value Y of the DC/DC converter. In a case that the battery systemfails to completely absorb the energy of the inverter, the DC/DC converterenters a saturation state and loses the control of the bus voltage. In this case, the bus voltage is automatically increased, and then the bus voltage is controlled by the inverter, that is, the bus voltage is controlled by the predetermined bus voltage value X of the inverter. In addition, if the battery systemis fully charged, the DC/DC converteris turned off. In an embodiment, the DC/DC converteris manually turned off. In another embodiment, the DC/DC converteris automatically turned off by the inverteror the local controller LC shown in, depending on the application environment. All the implementations fall within the protection scope of the present disclosure.

In practice, the two predetermined bus voltage values may be set separately, in order to ensure that the value X is greater than the value Y. In an embodiment, the predetermined bus voltage value Y of the DC/DC convertermay be set to a voltage for achieving the charging balance of the battery system, that is, a system balance voltage, such as, an MPPT voltage of the photovoltaic arrayconnected to the DC/DC converter. In addition, the inverterdetects the predetermined bus voltage value Y based on an MPPT strategy, and then transmits the predetermined bus voltage value Y to the DC/DC converter. In the MPPT strategy, operation parameters of the photovoltaic arrayare monitored and regulated in a real time manner to maximize the output power of the photovoltaic array, that is, the photovoltaic arrayalways operates at a maximum power point, thereby improving the energy conversion efficiency. The MPPT voltage is a voltage of the photovoltaic arrayoperating at the maximum power point. The predetermined bus voltage value X of the invertermay be positively correlated with the output power of the photovoltaic arrayconnected to the inverter, and may be a maximum system voltage. For example, the predetermined bus voltage value X of the inverteris set to a default value when the system is powered on. In an embodiment, the predetermined bus voltage value X of the inverteris set to a high voltage derating lower threshold of the inverter, and is regulated based on the output power of the photovoltaic arrayconnected to the inverter. In a case that the output power of the photovoltaic arrayconnected to the inverteris outside a preset fluctuation range including zero, the predetermined bus voltage value X of the inverteris only increased or only decreased with a preset step size based on a previous predetermined bus voltage value X. In a case that the output power of the photovoltaic arrayconnected to the inverteris within the preset fluctuation range, that is, the photovoltaic arrayhas a small output power or absorption power, the predetermined bus voltage value X is an open circuit voltage of the photovoltaic arrayconnected to the inverter. In this case, the bus voltage of the invertermay be controlled in an open constant voltage technology (OCVT) mode, that is, the predetermined bus voltage value X may be detected by the inverterbased on the OCVT strategy. In practice, the maximum system voltage is not limited to the high voltage derating lower threshold and the open circuit voltage, and may be a value between the two. In addition, the step size may be determined based on the output power rating of the photovoltaic array, which is not limited here, depending on the application environment.

Further, the predetermined bus voltage value X of the invertermay be set to be within a preset range, in order to prevent the predetermined bus voltage value X from being increased or decreased to be too large or too small. In practice, an upper limit and a lower limit of the preset range may be set based on the actual application condition. For example, the upper limit of the preset range may be set to the high voltage derating lower threshold of the inverter, thus avoiding the high voltage derating at the direct-current side of the inverter. The lower limit of the preset range may be set to an over-adjustment threshold of the inverter, thus avoiding over-adjustment. In practice, the high voltage derating lower threshold and the over-adjustment threshold are not limited, depending on the application environment. All the implementations fall within the protection scope of the present disclosure.

Through the OCVT strategy, the predetermined bus voltage value X may be controlled to be the maximum system voltage in a current condition. For example, the predetermined bus voltage value X is the high voltage derating lower threshold in an initial condition or under a condition of reaching an upper limit of adjustment, preventing the inverterfrom operating at power derating. In other cases, the energy obtained from the photovoltaic arrayis minimized, and the power grid energy is preferentially used, so as to greatly meet the power regulation demand at the alternating-current side.

In the foregoing solution, the battery systemis charged preferentially by using the power grid energy, improving the charging power and the response speed under a high frequency regulation demand such as FCAS. For example, the frequency regulation may be responded within one second, and thus this solution may be referred to as a one-second frequency regulation solution. Further, the bus voltage is controlled by the DC/DC converter, so that the remaining energy required by the battery systemcan be provided by the photovoltaic array. Further, the invertercompetes with the DC/DC converterfor the control of the bus voltage to achieve the corresponding energy scheduling.

It should be noted that, in a charging scenario, in a case that a charging power required by the battery systemis less than the power of the alternating-current side of the inverteror the battery systemis fully charged, excess energy outputted by the inverterflows into the photovoltaic array. The energy flowing into the photovoltaic panel in the photovoltaic arrayfor a long time may cause the photovoltaic panel to heat, affecting the efficiency of the photovoltaic panel, and even causing a serious damage to the photovoltaic panel.

In the foregoing solution according to the embodiment, in a case that the battery systemfails to completely absorb the power grid energy due to a small capacity of the battery system, limitation of state of charge (SoC) or the like, the bus voltage is raised based on the above principle to actively prevent the energy at the alternating-current side of the inverterfrom being transmitted to the direct-current side of the inverter, preventing the photovoltaic panel from being subjected to energy backflow, thereby protecting the photovoltaic panel.

It should be noted that the one-second frequency regulation solution is implemented by the invertercompeting with the DC/DC converterfor the control of the bus voltage, and a voltage for controlling a bus voltage (i.e., the predetermined bus voltage value X) of the inverteris greater than a voltage for controlling a bus voltage (i.e., the predetermined bus voltage value Y) of the DC/DC converter. The bus voltage is automatically raised as the available charging power of the DC/DC converteris decreased. The bus voltage may be raised to the predetermined bus voltage value X of the inverter. The DC/DC convertermay be turned off if the battery systemis fully charged. The bus voltage is automatically decreased as the available charging power of the DC/DC converteris increased. If the grid power provided by the inverteris decreased due to insufficient illumination or limited charging power at the alternating-current side of the inverter, the bus voltage is automatically stable at the predetermined bus voltage value Y of the DC/DC converter, such as a power balance point of the system. Therefore, this solution leads to a problem of reducing a frequency regulation capability for a photovoltaic over-distribution site due to the high voltage derating of the bus and the regulation saturation.

Therefore, another example in which the DC/DC convertercharges the battery systempreferentially by using the power grid energy in the direct-current coupling system is described below. For example, based on the direct-current coupling system shown into, as shown in(based on), the system further includes a direct-current switch S arranged between the photovoltaic arrayand the corresponding direct-current bus. Furthermore, each direct-current switch S connected to the corresponding direct-current bus is off during charging the battery system. In an embodiment, each direct-current switch S may be turned off when a power of a device connected to the direct-current switch S is zero in order to reduce a turn-off current of the direct-current switch S. For example, the invertermay be first controlled to operate at zero power, the DC/DC converterconnected to a same direct-current bus is on standby, and then the direct-current switch S connected to the direct-current bus is turned off.

In practice, the bus voltage of the inverteris controlled in a constant voltage technology (CVT) mode during charging the battery system. Before this, the bus voltage of the invertermay be controlled in a control mode (that is, an MPPT mode) under an MPPT strategy, or in the above OCVT mode, depending on the application environment. All the implementations fall within the protection scope of the present disclosure.

In addition, in the embodiment as described above, if the battery systemis fully charged, the DC/DC converterconnected to the battery systemmay be manually and/or automatically turned off.

In the solution, after entering the frequency regulation mode, the direct-current switch S connected to the inverteris first turned off, and then energy scheduling is performed by switching an outer voltage loop control mode (i.e., the above bus voltage control mode) to the CVT mode. The frequency regulation may be responded within six seconds, and thus the solution is referred to as a six-second frequency regulation solution. In addition, in this solution, the direct-current switch S is turned off, thereby preventing the photovoltaic panel from being subjected to energy backflow. Moreover, during frequency regulation scheduling in the CVT mode, the inverterautomatically regulates the charging power based on a demand of the battery system, thus solving the problems of high voltage derating of the bus and the regulation saturation without reducing the frequency regulation capability.

It should be noted that, the battery systemis charged preferentially by using the energy at the alternating-current side of the inverterbased on the one-second frequency regulation solution and the six-second frequency regulation solution. The one-second frequency regulation solution is that the charging power at the alternating-current side is actively reduced in the OCVT mode in a case that the battery systemfails to completely absorb the power grid energy, to protect the photovoltaic panel. The six-second frequency regulation solution is that the direct-current switch S is directly turned off, to protect the photovoltaic panel. Both the two solutions can solve the problem that the photovoltaic panel is subjected to energy backflow, thus protecting the photovoltaic panel. Moreover, the two solutions can meet the high frequency regulation demand of the Australian FCAS markets in the daytime, so that the invertercan complete the scheduling response of reverse switching of the maximum positive power and maximum negative power at the alternating-current side within 200 ms.

A charging control method for a direct-current coupling system is further provided according to another embodiment of the present disclosure, applied to the direct-current coupling system according to any one of the above embodiments. The photovoltaic array in the direct-current coupling system is connected to the inverter during charging the battery system, that is, the charging control method is applicable to the direct-current coupling system shown intofor the one-second frequency regulation solution.

As shown in, the charging control method includes the following steps Sto Son condition that the battery system is to be charged.

In step S, the inverter in the direct-current coupling system determines a second voltage for a charging balance of the battery system and a first voltage in a real time manner, and sends the second voltage to the DC/DC converter connected to the inverter.

The first voltage is greater than the second voltage.

In addition, the process of determining the first voltage in a real time manner may include: determining a maximum system voltage under a current condition as the first voltage. Detailed process of determining the maximum system voltage under the current condition as the first voltage includes the following stepstoshown in.

In step, a high voltage derating lower threshold of the inverter is determined as a default value of the first voltage.

In step, the first voltage is regulated in a real time manner based on an output power of the photovoltaic array connected to the inverter.

Detailed process of stepincludes as follows. A preset step size is determined based on an output power rating of the photovoltaic array connected to the inverter. In a case that the output power of the photovoltaic array connected to the inverter is greater than an upper limit of the preset fluctuation range including zero, a sum of the preset step size and a previous first voltage is determined as the first voltage. In a case that the output power of the photovoltaic array connected to the inverter is less than a lower limit of the preset fluctuation range, a value obtained by subtracting the preset step size from the previous first voltage is determined as the first voltage. In a case that the output power of the photovoltaic array connected to the inverter is within the preset fluctuation range including zero, an open circuit voltage of the photovoltaic array connected to the inverter is determined as the first voltage.

In practice, the output power rating of the photovoltaic array may be determined based on sampled electrical parameters on site. For example, a direction and a magnitude of the power of the photovoltaic array may be calculated from sampling results obtained by a voltage sensor and a current sensor on the direct-current side of the inverter, and the logic determination is performed based on the direction and the magnitude of the power of the photovoltaic array to determine the first voltage.

In step, the first voltage is limited within the preset range to obtain a limited value, and the limited value is determined as the first voltage.

The upper limit and the lower limit of the preset range may be referred to the above embodiments, which are not repeated herein.

In addition, the process of determining the second voltage for a charging balance of the battery system in a real time manner includes: detecting a MPPT voltage of the photovoltaic array connected to the DC/DC converter in a real time manner based on the MPPT strategy, and determining the MPPT voltage as the second voltage. The second voltage is merely an example and is not limited here, as long as the second voltage can achieve the charging balance of the battery system.